Add `.in` and `.txt` requirements files for BuildStream's pure python
dependencies. For each pair, the `.in` file is supposed to capture the
loose version requirements, and the corresponding `.txt` file is
supposed to have frozen requirements. We have 3 such sets:

* `requirements`: BuildStream's runtime dependencies
* `dev-requirements`: Dependencies for running tests
* `plugin-requirements`: Dependencies for core plugins

Note that the frozen requirements files will only be used for testing
purposes, and `setup.py` will continue to read loose requirements.

Add `tox.ini` file that will enable us to use
[tox](https://tox.readthedocs.io/) as a frontend for running tests.
Since we share the config via `setup.cfg` and requirements via
requirements files, commands like `python3 setup.py test` will continue
to work.

Instead of invoking tests throung `setup.py`, use `tox` as a frontend in
the CI pipelines.

Since we don't allow use of site packages by default in our `tox`
configuration, people will need to install non-python build dependencies
of some of our dependencies that do not provide pre-built wheels. We
have two such packages at the moment:

* psuitl: requires python C headers, compiler
* pygobject: requires python C headers, pkg-config, compiler, cairo
  headers, and gobject libraries